1. Field of the Invention
The present invention relates to a propylene-based resin composition and to a heat-shrinkable film using the resin composition. More particularly, the invention relates to a heat-shrinkable film superior in rigidity, heat shrinkage, weld-cut sealability and stretch processability and to a propylene-based resin composition capable of yielding such a heat-shrinkable film.
2. Description of the Related Art
A heat-shrinkable film is, in general, a film that is used in such a manner that a single object to be wrapped or an aggregate comprising a plurality of objects to be wrapped is wrapped with the heat-shrinkable film first and then the film is heated to shrink.
Such a heat-shrinkable is generally required to shrink at a temperature lower than the melting point of the film and to exhibit a high shrinkage. Furthermore, in recent years, the wrapping speed of automatic wrapping machines has been increased. Therefore, a heat-shrinkable film has come to be desired not to cause defective sealing, e.g. pinholes formed in a sealed portion, during weld-cut sealing employed in a wrapping process using an automatic wrapping machine. In addition, a heat-shrinkable film is desired to have a high rigidity and to be superior in stretch processability.
As an approach to improve a pinhole resistance after weld-cut sealing, JP, 10-7816,A discloses a method comprising addition of a nucleating agent to a polypropylene resin. JP, 2000-336221,A discloses a polypropylene resin composition that comprises a polypropylene resin which has an MFR of from 0.3 to 2.5 g/10 min and a flexural modulus of from 500 to 1000 MPa and a polypropylene resin which has a melting point ranging from 135 to 150xc2x0 C. and being higher than that of the former polypropylene by 5xc2x0 C. or more, an MFR of from 2.5 to 20 g/10 min and a flexural modulus of from 500 to 1000 MPa.
However, further improvement has been desired in rigidity, heat shrinkage and stretch processability as well as weld-cut sealability.
An object of the present invention is to provide a heat-shrinkable film superior in rigidity, heat shrinkage, weld-cut sealability and stretch processability and a propylene-based resin composition suitable as a raw material of the heat-shrinkable film. Another object of the present invention is to provide a heat-shrinkable film superior also in transparency in addition to the aforementioned four properties and a propylene-based resin composition suitable as a raw material of the heat-shrinkable film.
In a first aspect the present invention relates to a propylene-based resin composition comprising from 20 to 99 parts by weight of a propylene-based polymer A satisfying requirements (A-1) and (A-2) below and from 1 to 80 parts by weight of a propylene-based polymer B satisfying requirements (B-1) and (B-2) below, provided that the sum of the amounts of the propylene-based polymer A and the propylene-based polymer B is 100 parts by weight, wherein a melting point TmA of the propylene-based polymer A and a melting point TmB of the propylene-based polymer B satisfy requirement (C) below, wherein a melt flow rate MFRA of the propylene-based polymer A and a melt flow rate MFRB of the propylene-based polymer B satisfy requirement (D) below, and wherein the propylene-based resin composition satisfies requirements (E-1) and (E-2) below:
Requirement (A-1): A melt flow rate MFRA is from 0.3 to 20 g/10 minutes;
Requirement (A-2): A melting point TmA, which is defined as a peak temperature of a peak with a maximum intensity in a melting curve measured by DSC, is from 125 to 140xc2x0 C.;
Requirement (B-1): A melt flow rate MFRB is from 21 to 200 g/10 minutes;
Requirement (B-2): A melting point TmB, which is defined as a peak temperature of a peak with a maximum intensity in a melting curve measured by DSC, is from 135 to 170xc2x0 C.;
Requirement (C): A ratio of the melting point TmA of the propylene-based polymer A to the melting point TmB of the propylene-based polymer B, TmA/TmB, is less than 1;
Requirement (D): A ratio of the melt flow rate MFRA of the propylene-based polymer A to the melt flow rate MFRB of the propylene-based polymer B, MFRA/MFRB, is 0.01 less than MFRA/MFRB less than 1;
Requirement (E-1): A melt flow rate MFRC of the propylene-based resin composition is from 0.3 to 20 g/10 minutes; and
Requirement (E-2): A melting point TmC, which is defined as a peak temperature of a peak with a maximum intensity in a melting curve measured by DSC, of the propylene-based resin composition is from 130 to 145xc2x0 C.
In a second aspect the present invention relates to a heat-shrinkable film obtained by stretching the propylene-based resin composition at least uniaxially. This film is superior in rigidity, heat shrinkage, weld-cut sealability and stretch processablity.
In a third aspect the present invention relates to a propylene-based resin composition comprising from 20 to 99 parts by weight of a propylene-based polymer A satisfying requirements (A-1) and (A-2) mentioned previously, from 0.999 to 75 parts by weight of a propylene-based polymer B satisfying requirements (B-1) and (B-2) mentioned previously and from 0.001 to 5 parts by weight of a nucleating agent, provided that the sum of the amounts of the propylene-based polymer A, propylene-based polymer B and nucleating agent is 100 parts by weight, wherein a melting point TmA of the propylene-based polymer A and a melting point TmB of the propylene-based polymer B satisfy requirement (C) mentioned previously, wherein a melt flow rate MFRA of the propylene-based polymer A and a melt flow rate MFRB of the propylene-based polymer B satisfy requirement (D) mentioned previously, and where in the propylene-based resin composition satisfies requirement (E-3) below as well as requirements (E-1) and (E-2) mentioned previously:
Requirement (E-3): A crystallization rate of the propylene-based resin composition is from 1 to 400 seconds.
In a fourth aspect the present invention relates to a heat-shrinkable film obtained by stretching the propylene-based resin composition at least uniaxially. This film is superior in rigidity, heat shrinkage, weld-cut sealability and stretch processablity and also in transparency.